The present invention relates to a method of performing processing to form a cross section at a certain location on a semiconductor device, using a focused ion beam, for defect analysis or the like and of observing the cross section.
As LSIs (large-scale integrated circuits) and so on are fabricated with increasing device densities and in smaller size, techniques for processing and observing cross sections of the LSIs during development steps and fabrication steps with a focused ion beam device have been shown.
In particular, the position of a cross section to be processed is determined by the functions of a scanning ion microscope. Using the processed cross section as a plane, a rectangular hole is formed by maskless etching function. A desired cross-sectional portion is exposed, and then the sample is tilted to direct the cross-sectional portion in the direction of illumination of the ion beam. The processed cross section is observed again by the functions of the scanning ion microscope.
When the cross-sectional portion fabricated by the aforementioned method is observed by the functions of a scanning ion microscope, the ion current of the focused ion beam is set less than 5 pA to reduce the beam diameter. In this way, a high-resolution image is obtained. However, as the beam current decreases, the intensity of the secondary charged-particle signal also decreases, thus deteriorating the signal-to-noise ratio. Especially, the intensity of secondary ions is only approximately 1/10 to 1/1000 of the intensity of secondary electrons. Therefore, with scanning at an image refresh rate of a few or several seconds necessary for adjustment of the focus of the focused ion beam, the S/N is low. Image quality necessary for the work cannot be obtained. It has been difficult to finely adjust the voltage applied to the objective lens while watching the image of poor quality, to bring the focus of the focused ion beam onto the sample, and to obtain a high-resolution image of less than 10 nm. It is customary to slow the frame scanning rate as a method of improving the S/N. This method increases the time for which the secondary ion signal is accumulated to thereby improve the image quality. However, the image refresh rate is low. Therefore, skillfulness is necessary to adjust the focus of the focused ion beam while watching this image. It is not easy for everyone to make an adjustment.